Floating-gate transistor photodetector

ABSTRACT

A field effect transistor photodetector that can operate in room temperature includes a source electrode, a drain electrode, a channel to allow an electric current to flow between the drain and source electrodes, and a gate electrode to receive a bias voltage for controlling the current in the channel. The photodetector includes a light-absorbing material that absorbs light and traps electric charges. The light-absorbing material is configured to generate one or more charges upon absorbing light having a wavelength within a specified range and to hold the one or more charges. The one or more charges held in the light-absorbing material reduces the current flowing through the channel.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 14/214,513, filed on Mar. 14, 2014, which claims priority toU.S. Provisional Patent Application 61/783,108, filed on Mar. 14, 2013.The contents of the above applications are incorporated by reference intheir entirety.

TECHNICAL FIELD

This disclosure relates to floating-gate transistor photodetectors.

BACKGROUND

High sensitivity photodetectors and photon counters with a resolution atthe photon level are useful in many civilian and military applications,including quantum cryptography, imaging at low levels of ambientillumination, light detection and ranging (LIDAR), space exploration,and medical imaging. Examples of highly sensitive photodetectors includephotomultiplier tubes (PMTs), avalanche photodiodes (APDs), and quantumdot single-photon detectors (QDSPD). Nanomaterials, such as lead sulfidecolloidal quantum dots, cadmium selenide colloidal nano-particles, andzinc oxide (ZnO) nano-particles and nanowires, have been used in thephotodetectors.

SUMMARY

In general, in one aspect, a new type of solution-processed organicfield-effect transistor (OFET) as a highly sensitive un-cooledphotodetector is provided. The OFET integrates ZnO nano-particlespositioned between two dielectric layers under the channel layer, anduses the photo-generated, confined electrons to tune the channelcurrent. A large photoconductive gain and photon number memorizing andcounting capability are enabled by a novel super-float-gating mechanism.For example, a detectable ultraviolet light intensity of 2.6photons/μm²s(1.5×10⁻¹⁰ W/cm²) can be achieved.

In general, in another aspect, a field effect transistor photodetectoris provided. The photodetector includes a source electrode, a drainelectrode, a channel to allow an electric current to flow between thedrain and source electrodes, and a gate electrode to receive a biasvoltage for controlling the current in the channel. The photodetectorincludes a light-absorbing material that absorbs light and trapselectric charges, in which the light-absorbing material is configured togenerate one or more charges upon absorbing light having a wavelengthwithin a specified range and to hold the one or more charges, and inwhich the one or more charges held in the light-absorbing materialreduces the current flowing through the channel.

In general, in another aspect, a transistor photodetector includes asource electrode, a drain electrode, a channel layer to allow anelectric current to flow between the drain and source electrodes, a gateelectrode, a light-absorbing layer made of a material that absorbs lightand traps charges, the light-absorbing layer being disposed between thechannel layer and the gate electrode, a first dielectric layer disposedbetween the channel layer and the light-absorbing layer, and a seconddielectric layer disposed between the gate electrode and thelight-absorbing layer.

In general, in another aspect, a field effect transistor photodetectorincludes a source electrode, a drain electrode, a channel to allow anelectric current to flow between the drain and source electrodes, a gateelectrode to receive a bias voltage for controlling the current in thechannel, a dielectric layer disposed between the gate electrode and thechannel, and a window to allow light to illuminate the channel. Thechannel layer includes a light-absorbing material that absorbs light andtraps electric charges when a bias voltage is applied to the gateelectrode. The light-absorbing material is configured to generate one ormore charges upon absorbing light having a wavelength within a specifiedrange and to confine the one or more charges. The one or more chargesheld in the light-absorbing material increases the current flowingthrough the channel.

In general, in another aspect, a method of detecting photons includesapplying a voltage difference between a source electrode and a drainelectrode, and applying a bias voltage to a gate electrode, to cause anelectric current to flow from the source electrode through a channel tothe drain electrode; generating one or more electric charges by using alight-absorbing material to absorb light and generate the one or moreelectric charges; trapping the one or more electric charges within thelight-absorbing material; and reducing the current flowing in thechannel by using the trapped one or more electric charges in thelight-absorbing material to influence charge carriers in the channel.

In general, in another aspect, a method of detecting photons isprovided. The method includes providing a field effect transistorphotodetector that includes a source electrode, a drain electrode, achannel having a light-absorbing material, a gate electrode, and adielectric layer disposed between the gate electrode and the channel.The method includes applying a voltage difference between the source anddrain electrodes; applying a bias voltage to the gate electrode, inwhich initially negligible current flows in the channel when the biasvoltage is applied to the gate electrode; generating one or moreelectric charges in the channel by using the light-absorbing material inthe channel to absorb light and generate the one or more electriccharges; confining the one or more electric charges in the channel; andincreasing a current flowing in the channel upon confining the chargesin the channel.

Other aspects include other combinations of the features recited aboveand other features, expressed as methods, apparatus, systems, and inother ways.

Advantages of the aspects and implementations may include one or more ofthe following. The photodetectors can have high detectivity, operate inroom temperature without complicated cooling equipment, high resolution,good flexibility, integration with silicon technology, higher yield, andlower manufacturing/operating costs.

DESCRIPTION OF DRAWINGS

FIG. 1A is a diagram of the structure of a field effect transistorphotodetector.

FIG. 1B is an SEM cross-section image of the FET photodetector.

FIG. 1C is a graph showing the absorption spectrum of the ZnOnano-particle photoactive layer.

FIG. 1D is a diagram showing the detecting and resetting processes ofthe FET photodetector.

FIG. 2A is a graph showing the transfer characteristics of the FETphotodetector in the dark (red) and under UV illumination (blue).

FIG. 2B is a graph showing the FET photodetector detecting UV pulses andbeing reset using positive gate voltage pulses.

FIG. 2C is a graph showing exponential channel current decay of the FETphotodetector illuminated by various UV light intensities.

FIG. 2D is a graph showing the channel current decay rate as a functionof the incident light intensity ranging from 2.6 photons/μm²s to 3.5×10⁴photons/μm²s.

FIG. 2E includes a graph showing the decay rate of the FET photo sensorat a weak UV power intensity of 2.6 photons/μm²s and 16 photons/μm²s,respectively. Also shown is a graph showing the histograms of thecurrent decay rate at the corresponding UV light intensities.

FIG. 2F is a graph showing continuous photon counting of the FETphotodetector illuminated by UV (193 nm) pulses having a pulse width of12 nanosecond and interval of 200 ms.

FIG. 3A is a diagram illustrating the depth and cross-section of thepotential well caused by photo-induced confined electrons.

FIG. 3B is a graph showing calculated channel current decay rate fromthe enhanced floating-gate mechanism (dash line), and the measured decayrate of the FET photodetector as a function of the polystyrene layerthickness (triangle). The inset shows the cross-section and trap depthof the potential well as a function of the polystyrene layer thickness.

FIG. 3C is a graph showing variation of I_(SD) of the FET photodetectorwith different polystyrene layer thickness.

FIG. 4A shows a graph showing exponential channel current decay of theUV-IR FET photodetector illuminated by infrared light pulses having awavelength of 900 nm. The inset shows a diagram of the structure of aFET photodetector, in which a PbS nano-particle layer is insertedbetween the SiO₂ and the ZnO nano-particle layer.

FIG. 4B is a diagram showing the detecting process of the UV-IR FETphotodetector, in which the electrons are excited in the PbSnano-particle layer and then transported into the ZnO nano-particlelayer.

FIG. 5A is a diagram showing how incident photons from a scintillatortrigger the charge injection in the field effect transistor and tune theoutput current.

FIG. 5B is a photo of a flexible, transparent organic field effecttransistor array.

FIG. 6A is a diagram of the device structure of the C8-BTBT TTOFET.

FIG. 6B is an energy diagram of the device at source/semiconductorinterface in the dark with a bias voltage V_(G)>V_(T).

FIG. 6C is a diagram showing formation of trapped electrons andtriggered channel current in the TTOFET by UV light.

FIG. 6D is an energy diagram of the device at source/semiconductorinterface under light with a bias voltage V_(G)>V_(T);

FIG. 7A is a graph showing the channel current of the C8-BTBT TTOFET inthe dark and under UV light.

FIG. 7B is a graph showing the channel current of the C8-BTBT TTOFETunder a UV light ranging from 2 pW/cm² to 20 μW/cm².

FIG. 7C is a graph showing the channel current at a V_(sd) of −30 V as afunction of the incident light intensity.

FIG. 8A is a diagram of a double active layer structure with p-nheterojunction.

FIG. 8B is an energy diagram of the materials in the device of FIG. 8A.

FIG. 9 is a graph showing reduced dark current of the TTOFET by apositive gate bias to deplete the channel region.

FIG. 10 is a graph showing the low frequency noise spectral curves foran OFET device with a I_(SD) of about 1 μA.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

This disclosure describes a field effect transistor photodetector thatuses a light-absorbing, charge-confining material to absorb light andgenerate charges that are confined within the material, and uses theconfined charges to influence charged carriers in the channel of thetransistor to reduce the current flowing the channel. The amount ofchange of the channel current is dependent on the amount of chargesconfined in the light-absorbing, charge-confining material, and theamount of charges in the material is dependent on the number of photonsthat have been absorbed. There is a relationship between the amount ofchange in the channel current and the number of photons absorbed. Thus,it is possible to count the number of photons detected by measuring theamount of change in the channel current.

Referring to FIG. 1A, in some implementations, a field effect transistorphotodetector 100 includes a bottom-gate, top-contact type organic fieldeffect transistor. The transistor includes a source electrode 102, adrain electrode 104, a channel 106, and a gate electrode 108. The sourceand drain electrodes can be made of a conductive material, such as metalor indium tin oxide. In this example, a highly doped silicon wafer isused as both the substrate and the gate electrode. The gate electrodecan also be made of other conductive or semi-conductive materials, suchas indium tin oxide. A photoactive layer 110 is positioned between afirst dielectric layer 112 and a second dielectric layer 114. In theexample of FIG. 1A, the photoactive layer 110 is made of zinc oxide(ZnO), but the photoactive layer 110 can also be made of othermaterials, such as lead sulfide. The material in the photoactive layer110 can be in the form of e.g., nano-particles, nano-wires, but is notlimited to these forms. More than one type of photoactive material canbe used. Two or more photoactive materials can be mixed together or bein separate layers. The photosensitive material(s) can be selected basedon the wavelength of light to be detected. For example, zinc oxide canbe used in a sensor for detecting UV photons, and lead sulfide can beused in a sensor for detecting infrared and visible light photons. Asensor having both zinc oxide and lead sulfide can detect light rangingfrom UV to infrared wavelengths. The photoactive layer 110 can have athickness in a range from, e.g., 1 nm to several hundred nanometers, butother thicknesses can also be used.

The bottom dielectric layer 112 can be a thermal growth silicon oxide(SiO₂) layer having a thickness of 200 nm. The top dielectric layer 114can be a thin layer of polystyrene (PS). In the example of FIG. 1A, thechannel layer 106 is an organic semiconductor layer made of eithertrimethyl-[2,]quarter-thiophen-5-yl-silane (4T-TMS) or pentacene. Othermaterials, such as graphene, can also be used for the channel layer. Thethin polystyrene dielectric layer can be formed by the vertical phaseseparation of a 4T-TMS:PS blend after drop coating. The polystyrenelayer 114 forms an energy barrier between the photoactive layer 110 andthe semiconductor channel layer 106 so that the charges in thephotoactive layer 110 does not migrate to the channel layer 106.

FIG. 1B shows a scanning electron microscope (SEM) cross-section image120 of a field effect transistor photodetector. The ZnO nano-particlelayer, the polystyrene layer, and 4T-TMS layer can be identified in theimage 120. The optical response spectrum of the sensor is determined bythe ZnO nano-particle layer.

FIG. 1C is a graph 130 showing the absorption spectrum of the ZnOnano-particle layer having a thickness of 60 nm, in which an opticalband gap of 3.4 eV for the ZnO nano-particles can be identified. Thethickness of the polystyrene layer was controlled to be around 10 nm.The thickness of the polystyrene layer can be adjusted by changing theratio of the 4T-TMS:PS blend.

Referring to FIG. 1D, the working process of the field effect transistorphotodetector 100 can be described in three stages. The photodetector100 works when the transistor is in the ON state. In an initial firststage 140, the sensor 100 is in the dark, the ZnO nano-particle layerand the polystyrene layer, acts as insulating dielectric layers. Theholes 142 injected from the source electrode 102 are built up at the4T-TMS/PS interface 144 because of the insulating property of thepolystyrene and the large energy difference between the highest occupiedmolecular orbital (HOMO) of 4T-TMS (−5.3 eV) and the valence band of ZnO(−7.6 eV). Driven by the source-drain voltage, the carriers transportlaterally to the drain electrode 104.

In a detecting stage 150, the photodetector 100 absorbs one or morephotons. The absorption of UV photons by 4T-TMS and polystyrene is smalldue to their large band gap and small thickness. Incident UV photonsexcite electron-hole pairs in the ZnO nano-particle layer 110. Anegative bias voltage is applied to the gate electrode 108, which causesthe electrons to sweep crossing the ZnO nano-particles layer 110 and beconfined at the ZnO/PS interface 146. The confined electrons 152 at theZnO/PS interface 146 impose a trapping effect to the transporting holecarriers 142 in the nearby semiconductor channel 106 by columbicattraction, and thus results in a reduced source-drain output current(ΔI_(SD)) in the organic field effect transistor. The trapping effect isrepresented by a trap 154 in the diagram. The reduction in thesource-drain output current ΔI_(SD) is correlated to the density ofphoto-generated confined electrons 152 at the ZnO/PS interface 146, andthus that of incident light intensity.

The confined electrons in the ZnO layer influence the current flowingthrough the channel. In this case, the confined electrons reduce theamount of current flowing through the channel. Thus, the ZnO layerfunctions as a “floating gate.” As described below, the trapping effectgenerated by the confined electrons causes the channel current todecrease exponentially in relation to the amount of photons absorbed bythe ZnO layer, thus the ZnO layer is referred to as an “enhancedfloating-gate” mechanism.

The confined electrons 152 at the ZnO/PS interface 146 can be held for along time (e.g., several minutes) by the applied gate electric fieldwithout recombination. Thus, the reduction in the source-drain currentΔI_(SD) persists even after the incident light is turned off. This isone of the differences between the field effect transistor photodetector100 and conventional photoconductor or diodes in which the currentsignal decays quickly once the light is turned off. In the field effecttransistor photodetector, the reduction in the source-drain currentΔI_(SD) is determined by the amount of absorbed photons rather than theintensity of the light, so that the device works in a photon-countingmode.

The photodetector 100 can operate as a photon counter that countsincident photons continuously, and can be reset by a reversed gate biaspulse. In a resetting stage 160, a reverse bias voltage is applied tothe gate electrode 108, which causes the electrons 152 at the ZnO/PSinterface 146 to move toward the holes 162 in the ZnO nano-particlelayer 110 and recombine with them. As a result, the photon-induced trapsdisappear and the channel current resumes its initial value.

FIG. 2A is a graph 170 showing example transfer curves of the fieldeffect transistor photodetector 100 in the dark and under UVillumination. The layer of ZnO nano-particles is covered with a thinlayer of polystyrene for a number of reasons. One of functions of thepolystyrene layer is to flatten and encapsulate the surface of the ZnOnano-particle layer so that the hole carriers are not trapped by thesurface defects on the ZnO nano-particles and the field effecttransistor photodetector can work as a normal transistor device in thedark. The calculated hole mobility of the 4T-TMS-based transistor in thedark is about 0.065 cm²/Vs. When the field effect transistorphotodetector was illuminated by UV light, the channel current droppedimmediately. No channel current change was observed when the channelregion was illuminated by visible light (400˜700 nm), which confirmsthat the current variation of the field effect transistor was caused bythe absorption of UV light by the large band gap of ZnO nano-particles.

The photon detecting and resetting of the field effect transistorphotodetector was demonstrated by recording the ΔI_(SD) under theillumination of a train of UV light pulses, which was followed by areversed gate bias pulse after each light pulse. Referring to FIG. 2B, agraph 180 shows a UV light pulse 182 with a duration of 0.5 secondscaused a change in drain-source current ΔI_(SD) 184 from 700 nA to 5 nA.A reverse bias pulse 186 having about 40 ms pulse width is applied torecover the drain-source current I_(SD) to its initial value so that thefield effect transistor photodetector 100 has an “opticalwrite/electrical reset” working process. The field effect transistorphotodetector 100 can also be partially reset by just turning off thegate voltage, because turning off the gate bias releases the confinedelectrons to the ZnO layer. An additional positive gate voltage leads toa fast and complete resetting. The detector shows increased resistance(decreased current) by illumination, which can be measured by (i)read-out circuits with mirror circuit scheme for the currentcancellation, or (ii) read-out circuits that can convert the deviceresistance change to voltage output change.

Referring to FIG. 2C, to evaluate the sensitivity and linear dynamicrange of the field effect transistor photodetector 100, the variationsof I_(SD) under different UV light intensity were studied, where thedifferent UV light intensities were generated by a gallium nitride (GaN)light emitting diode (LED) (emission peak at 345 nm) and attenuated witha set of neutral density filters. As shown in graph 190, under aconstant UV light intensity, the I_(SD) decreased exponentially withtime because the FET photodetector continued to absorb and countphotons. A stronger UV intensity results in a faster decay rate (decayrate is defined as k_(SD)=d log(ΔI_(SD))/dt). There is a backgroundI_(SD) decay of the FET photodetector in the dark, whose fluctuationdetermines the lowest light intensity that can be measured (or noiseequivalent power, (NEP)).

Referring to FIG. 2D, for a better understanding of the relationshipbetween the current decay rates and the UV light intensity, we plottedthe k_(SD) obtained as a function of the UV intensity, as shown in graph200. The k_(SD) linearly increases with the incident UV light intensityin over four orders of magnitude from 2.6 photons/μm²s to 3.5×10⁴photons/μm²s.

Referring to FIG. 2E, a graph 210 shows the k_(SD) of the field effecttransistor photodetector under a rectangular UV light pulse having alight intensity of 2.6 photons/μm²s and 16 photons/μm²s, respectively,demonstrating the capability of the photodetector in weak lightdetecting. The distribution of the current decay rate in the dark orunder weak UV light (2.6 photons/μm²s and 16 photons/μm²s, respectively)is shown in FIG. 2E. In some examples, at the minimum detectable powerintensity level, the photodetector performs an integration for severalseconds (e.g., tens of seconds) to distinguish the signal from noise(corresponding to about 100 photons/μm²). The minimum detectable lightintensity for the FET photodetector can be very low because of the highgain from the enhanced floating-gate mechanism. The high sensitivity ofthe photodetector can be attributed to the large photoconductive gain(the result of a sufficient supply of charges provided by the sourceelectrode) and the current change due to the photo-induced conductivitychange. The long recombination lifetime of the confined electrons caninduce a large gain in the FET photodetector.

Referring to FIG. 2F, to demonstrate the light-pulse-counting capabilityof the FET photodetector, the ΔI_(SD) under a train of nano-second UVpulses was recorded and shown in a graph 220. The UV light pulse train,with a pulse width of 12 ns and an interval of 200 ms, was generated byan excimer UV laser (BraggStar Industrial LN 1000). Since no recoveryperiod is required after each photon event, the FET photodetector 100can count photons continuously without dead time. The I_(SD) decreasesstep by step upon each incident light pulse, and the I_(SD) keepsconstant between each light pulse. This unique device behavior enablesthe counting of light pulses with a single device without sophisticatedelectronics, which may be useful in optical communication applications.In this example, the device response time is shorter than 30 ms, whichis limited by our current measurement system. The unique “memory”function of the FET photodetector 100 enables the separation of lightdetection and signal readout so that the electronics do not need to havea quick read out speed, which can be important in applications where thetotal incident photon numbers are concerned, such as radiationdetection.

The following describes the gating mechanism of the FET photodetector100. The large photoconductive gain in the FET photodetector 100generates a large signal output (ΔI_(SD)) per incident photon andenables the weak light detection near the single-photon level. A linearvariation of transistor output current with the incident photons can beexpected if the charged ZnO nano-particles only function as a floatinggate to tune the apparent gate bias to the semiconductor layer:

$\begin{matrix}{{\Delta \; I_{SD}} = {g_{m}\frac{qD}{ɛ}N_{p\; h}}} & \left( {{Equ}.\mspace{14mu} 1} \right)\end{matrix}$

where g_(m) is the transconductance of the transistor at a fixed gatebias, q is the element charge, D is the distance between the gate andthe photon absorption layer, s is the electric permittivity of the spacematerials, and N_(ph) is number of absorbed photons. However, the FETphotodetector 100 has an exponential dependence of ΔI_(SD) with theamount of absorbed photons: ΔI_(SD)∝exp(−N_(ph)). It is then expectedthat the charged ZnO nano-particles near the 4T-TMS/PS interface haveother roles in addition to their function as a floating gate to changethe effective gate bias. To better understand the high sensitivity ofthe FET photodetector 100 at room temperature, the following is a devicemodel that can be used to explain the exponential decrease of thecurrent under constant illumination.

Referring to FIG. 3A, a diagram 230 shows the influence of the confinedelectrons at the polystyrene/ZnO interface 146 on the transporting holesin the channel layer 106. The exponential I_(SD) decays can be explainedby the trap-induced carrier mobility loss in the organic semiconductorchannel of the field effect transistor. The I_(SD) through the fieldeffect transistor in the saturation regime can be described by

$\begin{matrix}{I_{SD} = {\frac{\mu \; C_{i}W}{2L}\left( {V_{g} - V_{t}} \right)^{2}}} & \left( {{Equ}.\mspace{14mu} 2} \right)\end{matrix}$

where μ, C_(i), W, L, V_(g), and V_(t) are the hole mobility, specificdielectric capacitance, channel width, channel length, gate bias, andthreshold voltage, respectively. The mobility of the carriers (holes in4T-TMS) is sensitive to the traps with an exponential dependence,

μ∝exp(−ΔE _(tr) /kT)  (Equ. 3)

where ΔE_(tr) is the average energy trap depth caused by the columbicinteraction between the confined electrons at the polystyrene/ZnOinterface 146 and the transporting holes in the channel layer 106. Thesensitive response of the carrier mobility in the semiconductor channelto energy traps is due to the columbic interaction of the channelcarriers and the confined charges. The columbic interaction is importantin changing the current in the channel of FET photodetector because of(i) the very thin separation layer (in the example of FIG. 1A, thepolystyrene layer 114) between the channel layer 106 and the ZnOnano-particle layer 110, which is the thickness of polystyrene layer onthe order of about 5 to 20 nm, and (ii) the low dielectric constant ofpolystyrene (∈_(r)=2.6).

Each confined electron at the polystyrene/ZnO interface 146 imposes apotential well for the transporting hole carriers in the semiconductorchannel 106 due to the columbic force between them. The average trapdepth by all of the generated traps is expected to be proportional toincident photons, or generated traps density n_(tr)(t) at theZnO/polystyrene interface 146, as well as the trap depth of eachindividual trap (ΔE_(max)):

ΔE _(tr)(t)=cΔE _(max) S _(c) n _(tr)(t)  (Equ. 4)

where n_(tr)(t)=aPt, which was determinate by UV light intensity (P, ina unit of photons/μm²s), the illuminating time (t), and the quantumefficiency (a) of the trap formation by the incident photons. Theparameter S_(c) is the cross-section of each trap as shown in FIG. 3A,and c is a constant describing how each individual trap contributes tothe average trap depth. Therefore, one can derive

$\begin{matrix}{{I_{SD}(t)} = {I_{0}{\exp \left( {{- \frac{a\; c\; \Delta \; E_{{ma}\; x}S_{c}}{kT}}{Pt}} \right)}}} & \left( {{Equ}.\mspace{14mu} 5} \right)\end{matrix}$

where I₀ is the initial channel current. From Equation 5, we candetermine the reason for the exponential decrease of the channel currentunder constant UV illumination and that the decay rate k_(SD) isproportional to the UV light intensity (k_(SD)=(acΔE_(max)S_(c)/kT)P).It is consistent with the experimental results shown in FIGS. 2B to 2E.

Based on the enhanced floating-gate mechanism, the thickness of thepolystyrene layer 114 has an important role in determining thesensitivity of the FET photodetector 100. At a given trap density, adecreased polystyrene thickness generates an increased hole capturecross-section area and an increased average trap depth, as shown in FIG.3A, both increasing k_(SD).

Referring to FIG. 3B, the S_(c), ΔE_(max), and k_(SD) of the deviceswith varied polystyrene thicknesses, from 30 nm to 5 nm, were calculatedand are shown in a graph 240. As shown in FIG. 3B, the current decayrate of the FET photodetector 100 (which represents the sensitivity ofthe photodetector) increases significantly when the polystyrenethickness is less than 20 nm.

To verify the simulation result shown in FIG. 3B, a series of deviceswith varied polystyrene thicknesses were fabricated. For a bettercontrol of the polystyrene thickness, the polystyrene layer was spincoated from a pure polystyrene solution. The semiconducting layers(pentacene) were thermally deposited on the polystyrene surface.

Referring to FIG. 3C, the current decay of the devices with differentpolystyrene thicknesses under a UV light of 3 μW/cm² is shown in a graph250, and the device output current decay rate is plotted in FIG. 3B. Theresults show that the device performance is very sensitive to thethickness of the polystyrene layer. The experimental results agree withthe predicted k_(SD) well with a single fitting parameter c of 0.013.

The FET photo sensor 100 shows a weak response under UV light when thepolystyrene layer 114 exceeds 30 nm, probably because other mechanismsalso contribute to the detection, such as a regular floating-gatemechanism. The current changes due to the regular floating-gatemechanism estimated from Equation 1 are also shown in FIG. 3C, and isten times lower than the enhanced floating-gate mechanism when thepolystyrene is thinner than 10 nm. This analysis shows how the thicknessof the polystyrene layer 114 determines the device's sensitivity. Thetransistor channel current is determined by the channel width to lengthratio (W/L) rather than channel area, and the output current decay ratek_(SD) is independent of the device area. The signal (ΔI_(SD)) of FETphoton sensor 100 does not scale with device area, which is a usefulfeature for the application of photodetector arrays in which a highresolution can be achieved by scaling down the device size withoutcompromising sensitivity.

The following describes the effect of trapped electrons, and an infraredphotodetector is demonstrated. To further confirm the enhancedfloating-gate mechanism and the universal application of such a devicestructure, an UV and infrared (UV-IR) photodetector based on samemechanism was fabricated. Lead sulfide (PbS) nano-particles weresynthesized with a tunable size from 2 to 6 nm which extended theabsorption of the active layer from UV to near infrared region. PbSnano-particles with sizes of 3-4 nm, which have an absorption cut-off of1,150 nm and a band gap of about 1.1 eV, were mainly used in thisexample to demonstrate the working principle of infrared photodetectors,although PbS nano-particles of other sizes can also be used.

If ZnO nano-particles are replaced by lead sulfide (PbS) nano-particlesin the device structure shown in FIG. 1A, the photodetector may notfunction well. It is possible that the lower LUMO of PbS nano-particlesallows the injection of holes from 4T-TMS into PbS under the strong gateelectric field and damaged the channel transport path. Another possiblecause of the device failure may be the incompatible interface of PbSnano-particles layer with polystyrene dielectric.

Referring to FIG. 4A, a FET photodetector 260 that can detect UV andinfrared photons includes a layer of ZnO nano-particles and a layer ofPbS nano-particles 264, both of which are positioned between a layer ofSiO₂ 266 and a layer of polystyrene 268. The ZnOnano-particles/polystyrene interface in FET photodetector 260 is similarto that of the FET photodetector 100 in FIG. 1A, so the sensor 260 has asimilar response to UV light as the sensor 100.

Referring to FIG. 4B, under infrared illumination having a wavelength of900 nm, the ZnO nano-particles are not excited directly due to itslarger band gap, while electrons can be confined at the ZnO/polystyreneinterface by the photo-induced electron transfer from PbS to ZnOnano-particles. Therefore, in addition to detecting UV light, the FETphotodetector 260 can also detect infrared light.

As shown in FIG. 4A, channel current reduction can be observed when theFET photodetector 260 was illuminated by infrared light. This shows theenhanced floating-gate mechanism can be used in infrared sensitivephotodetectors. It is likely that the trapping effect caused by theconfined electrons in the ZnO nano-particle resulted in the observedcurrent reduction.

To determine the stability of the FET photodetector 100, the deviceilluminated with UV light having an intensity of 100 nW/cm² for 200hours. The test results indicate that the photodetector 100 does notshow visible decay.

We have described a novel enhanced floating-gate transistor for photondetection and counting with high sensitivity at room temperature. TheFET photodetector 100 has an enhanced floating-gate mechanism. Theincident photons induce confined electrons beneath the channel layerwhich tune the current flowing through the transistor channel. In someexamples, the photodetector can detect UV light intensity of 2.6photons/μm²s (0.15 nW/cm²). The unique memory-like photodetectingprocess enables the FET photodetector to count the photons without deadtime. A small spacing between the ZnO nano-particles and the channelregion is important for the high device sensitivity observed. The FETphotodetector can be used for un-cooled, low bias, low-cost,high-resolution photodetector arrays or photon-manipulated computation.

The following describes methods for fabricating the FET photodetector100. In some implementations, highly arsenic-doped silicon having aresistivity of, e.g., 0.001 to 0.005 ohm/cm is used as the gateelectrode, which is covered by a layer of thermal-grown silicon oxide(SiO₂) having a thickness of about, e.g., 200 nm. After UV-ozonetreatment of the SiO₂ surface, a ZnO nano-particle layer having athickness of about, e.g., 60 nm is spin coated from a ZnO:chlorobenzene(e.g., 2.5 wt %) solution at, e.g., 3000 rpm for, e.g., 40 seconds. TheZnO nano-particle layer is thermally annealed in the air at, e.g., 260°C. for about, e.g., 30 minutes.

In some implementations, for the fabrication of the semiconductorchannel layer, trimethyl-[2,5′5′,2″,5″,2″,]quarter-thiophen-5-yl-silane(4T-TMS) and polystyrene (e.g., 9:1 by weight) are dissolved in 1,2-dichlorobenzene (DCB) (e.g., 4 mg/ml in all). Then the solution isdrop coated on the ZnO surface, during which the substrate is located ona tilted hotplate. The tilting angle can be, e.g., 2.5°, and the dryingtemperature can be 80° C. During the drying process, there is a verticalphase separation between the polystyrene and 4T-TMS components, whichresults in a bilayer structure of polystyrene/4T-TMS with thepolystyrene thin film attached on the ZnO surface. Gold (Au) source anddrain electrodes are thermal evaporated with a channel length and widthof 100 μm and 1 mm, respectively. The electrical characteristics of thedevices can be measured using, e.g., two Keithley 2400 Source Meters inambient conditions.

To test the FET photodetector, UV light can be generated from deep UVlight emitting diodes (LED) having a wavelength of 345 nm (e.g.,UVTOP®345TO39/TO18FW, Sensor Electronic Technology, Inc.). Thephotodetector and the UV LED can be placed in a metal box to exclude theambient light. The UV intensity can be controlled by changing thedriving current of the diodes and using neutral filters. The incidentlight intensity can be calibrated with a UV photodetector beforeapplying the filters.

The following describes modeling of device sensitivity versuspolystyrene thickness. The influence of the polystyrene thickness (d) onthe decay rate of the device current under illumination can be estimatedas follows.

The cross-section of the photon-induced high resistance region (shown inFIG. 3A) is:

S _(c) =πr _(c) ²  (Equ. 6)

Here, a critical boundary is defined for the trapping cross-sectionhaving a radius of r_(c) in which the thermal activation energy ofelectrons is no more than the potential depth, i.e., q/4π∈_(r)∈₀√{squareroot over (d²+r²)}≧kT, where ∈_(r) is the relative dielectric constantof polystyrene (2.6), ∈₀ is the dielectric constant of vacuum, r is thehorizontal distance between the hole and the confined electron, k is theBoltzmann constant, and T is the room temperature. A reduced d resultedin an increased r_(c) and, hence, an increased S_(c).

A reduced polystyrene thickness results in deeper traps which causelarger velocity loss of transporting holes. The maximum value of thedepth is:

ΔE _(max) =q/4π∈_(r)∈₀ d  (Equ. 7)

The decay rates k_(SD) of the FET photodetector 100 can be determinedfrom Equations 5 to 7 as shown in FIG. 3B, in which the trap formationefficiency a (38%) is assumed to be equal to the absorbance at 345 nm(FIG. 1D).

A light sensor can include the FET photodetector 100 or 260, and acontrol circuit to control the operation of the FET photodetector. Thecontrol circuit may provide bias voltages to the drain, source, and gateelectrodes. The control circuit may provide the reset gate pulses 186shown in FIG. 2B. The control circuit may include a readout circuit todetermine the amount of change in the channel current, and determine thenumber of photons detected based on the current change. The FETphotodetector 100 and 260 can be calibrated to establish a relationshipbetween the number of photons detected and the amount of current change.The calibrated values can be stored in a lookup table.

An image or video sensor can include an array of pixels in which eachpixel includes a FET photodetectors 110 (or 260). Such image or videosensor can be used to generate images or video at very low lightenvironments. For example, to use the FET photodetector in a cameraimage sensor, a controller generates a bias voltage that is applied tothe gate electrode. The camera shutter is opened for a certain amount oftime, and the change in the channel current is measured. Based on apredetermined relationship between the amount of light detected and thechange in the channel current, the light intensity at each FETphotodetector can be determined based on the amount of channel currentchange. After an image is read from the array of pixels, the controllergenerates a reverses bias voltage that is applied to the gate electrodeto reset the photodetectors.

In some implementations, an image sensor can have several FETphotodetectors in which some of the photodetectors can detect UV light,and some of the photodetectors can detect infrared light and/or visiblelight. This way, the image sensor can detect a wide range of lightwavelengths. An image sensor can also have several FET photodetectors inwhich each photodetector includes two or more photoactive materials thatin combination enables the photodetector to detect a wide range of lightwavelengths.

As described above, the structure of enhanced floating-gatephotodetector is based on a field-effect transistor, which includes asource electrode, a drain electrode, a gate electrode, compositedielectric layer(s), and composite conducting layer(s). In addition,optical coupling structures and interfacial buffer layers can becombined.

The FET photodetector 100 can be modified in various ways. For example,the field effect transistor can be a lateral field effect transistor. Alateral FET can be a bottom-gate/top-contact FET, abottom-gate/bottom-contact FET, a top-gate/top-contact FET, or atop-gate/bottom-contact FET. A vertical field effect transistor can alsobe used.

The source and drain electrodes can be electron-injecting typeelectrodes, and the electrodes can be made of, for example, magnesium,aluminum, calcium, lithium, sodium, potassium, strontium, cesium,barium, iron, cobalt, nickel, copper, silver, zinc, tin, samarium,ytterbium, chromium, gold, grapheme, alkali metal fluoridealkaline-earth metal fluoride, alkali metal chloride, alkaline-earthmetal chloride, alkali metal oxide, alkaline-earth metal oxide, metalcarbonate, metal acetate, n-type silicon (n-Si), or a combination of theabove. The source and drain electrodes can also be hole-injecting typeelectrodes, and the electrodes can be made of, for example, indium-tinoxide (ITO), indium zinc oxide (IZO), silver, gold, platinum, copper,chromium, indium oxide, zinc oxide, tin oxide, polyaniline (PANT) basedconducting polymer, 3,4-polyethylenedioxythiopene-polystyrenesultonate(PEDOT) based conducting polymer, carbon nanotube (CNT), graphite,grapheme, molybdenum oxide, tungsten oxide, vanadium oxide, silveroxide, aluminum oxide, p-type silicon (p-Si), or a combination of theabove.

The material for the gate electrode can be metal, for example,magnesium, aluminum, calcium, lithium, sodium, potassium, strontium,cesium, barium, iron, cobalt, nickel, copper, silver, zinc, tin,samarium, ytterbium, chromium, gold, alkali metal, silver, gold,platinum, copper, chromium, or a combination of the above. The gateelectrode can also be made of an inorganic semiconductor, for example,electrically doped Si, germanium, III-V semiconductors, II-VIsemiconductors, ITO, IZO, indium oxide, zinc oxide, tin oxide,molybdenum oxide, tungsten oxide, vanadium oxide, silver oxide, aluminumoxide or combinations thereof. The gate electrode can be made of organicmaterials, for example, polyaniline (PANT) based conducting polymer,3,4-polyethylenedioxythiopene-polystyrenesultonate (PEDOT) basedconducting polymer, carbon nanotube (CNT), graphite, graphene, or acombination of the above.

The insulating dielectric layer can be an inorganic dielectric material,for example, silicon oxide (SiO_(x)), silicon nitrides (SiN_(x)),aluminum oxide (AlO_(x)), tantalum oxide, titanium oxide, hafnium oxide,zirconium oxide, cerium oxide, barium titanate (BaTiO₃), bariumzirconate titanate (BZT), barium strontium titanate (BST), leadzirconate titanate (PZT). The dielectric layer can be an organicdielectric material with groups including polystyrene (PS),polymethylmethacrylate (PMMA), poly(4-methoxyphenylacrylate) (PMPA),poly(phenylacrylate) (PPA), poly(2,2,2-trifluoroethyl methacrylate)(PTFMA), polyvinyl alcohol (PVA), cyanoethylpullulan (CYEPL), polyvinylchloride (PVC), poly-4-vinylphenol (PVP), cross-linked PVP, PVPcopolymer, benzocyclobutene (BCB), poly(ethylene terephthalate) (PET),polyvinylacetate (PVAc), polyvinylidene fluoride (PVDF),polychlorotrifluoroethylene, polytetrafluoroethylene (PTFE), polyimide,polyester, polynorbornene, perylene, or a combination of the above. Thedielectric layer can be made of polymeric-nanoparticle (NPs) composites,for example, the polymer mentioned above combining with TiO₂ NPs, BaTiO₃NPs, Al₂O₃ nano-particles.

The photoactive layer can be a single pristine film, a mixed film, or astacked film. The photoactive materials can be embedded in otherinsulating and/or semiconducting matrixes. The thickness of each layercan be from, e.g., 1 nm to 10 μm.

The photoactive materials can be in the form of photo activenano-particles, nano-rods, or nano-wires. The materials include zincoxide (ZnO), titanium oxide (TiO_(x)), tin oxide (SnO_(x)), indium oxide(InO_(x)), copper oxide (Cu₂O), zinc sulfide (ZnS), cadmium sulfide(CdS), lead sulfides (PbS), iron pyrite (FeS₂), cadmium selenide (CdSe),lead selenide (PbSe), cadmium telluride (CdTe), lead telluride (PbTe),silicon (Si), germanium (Ge), gallium nitride (GaN), gallium arsenide(GaAs), indium arsenide (InAs), indium antimonide (InSb),Pb_(1-x)Sn_(x)Te, Hg_(1-x)Cd_(x)Te, InAsSb, InTlSb. The photoactivematerial can include super lattices, e.g., InAs/GaInSb, HgTe/CdTe. Thephotoactive material can be an organic material with conjugatedπ-electronic systems, e.g., including TiO_(x): phthalocyaninederivatives, naphthalocyanine derivatives, porphyrin derivatives,perylene derivatives, coumarin derivatives, rhodamine derivatives, eosinderivatives, erythrosine derivatives, acenes and polyacenes derivatives,oligothiophenes derivatives, benzothiophene (BT) derivatives,benzothiadiazole derivatives, benzodithiophene (BDT), fullerenederivative (e.g. C60, carbon nanotube, graphene and etc.), perylenederivative, polythiophene (PT) derivatives, polycarbazole or itsderivatives, poly(p-phenylene vinylene) (PPV) or its derivatives,polyfluorene (PF) or its derivatives, cyclopentadithiophene basedpolymers, benzodithiophene (BDT) based polymers, or a combination of twoor more of the above materials.

The ranges of wavelengths of light (λ) that can be absorbed by thematerials are listed below:

ZnO: λ<370;

TiOx: λ<390 nm;

SnOx: λ<320 nm;

InOx: λ<420 nm;

Cu₂O: λ<600 nm;

ZnS, λ<360 nm;

CdS, λ<520 nm;

PbS, λ<3300 nm;

FeS2, λ<1.6 μm;

CdSe, λ<720 nm;

PbSe, λ<4.6 μm;

CdTe, λ<830 nm;

PbTe, λ<5.0 μm;

Si, λ<1.1 μm;

Ge, λ<1.9 μm;

InAs, λ<3.5 μm;

GaAs, λ<870 nm;

GaN, λ<370 nm;

InSb, λ<7.3 μm;

Pb_(1-x)Sn_(x)Te, λ<6.0 μm;

Hg_(1-x)Cd_(x)Te, λ<12 μm;

InAsSb, λ<10 μm;

InTlSb, λ<8.5 μm;

organic semiconductors: 250 nm<λ<750 nm.

Thus, when different materials are used in the FET photodetector, thephotodetector can be used to detect different wavelengths of light.

The composite conducting layer(s) can be single pristine film, mixedfilm, or stacked film.

The conducting materials can be small molecular or polymer conjugatingsemiconductors, including phthalocyanine derivatives, naphthalocyaninederivatives, porphyrin derivatives, perylene derivatives, coumarinderivatives, rhodamine derivatives, eosin derivatives, erythrosinederivatives, acenes and polyacenes derivatives, oligothiophenesderivatives, benzothiophene (BT) derivatives, benzothiadiazolederivatives, benzodithiophene (BDT), fullerene derivative (e.g. C60,carbon nanotube, graphene and etc.), perylene derivative, polythiophene(PT) derivatives, polycarbazole or its derivatives, poly(p-phenylenevinylene) (PPV) or its derivatives, polyfluorene (PF) or itsderivatives, cyclopentadithiophene based polymers, benzodithiophene(BDT) based polymers, or a combination of two or more of the abovematerials.

In the example of FIG. 1A, the channel layer is made of P-typesemiconductor, and a negative bias voltage is applied to the gateelectrode during the photodetecting mode. In some implementations, thechannel layer is made of N-type semiconductor, and a positive biasvoltage is applied to the gate electrode during the photodetecting mode.

The technique of using confined charges to influence channel current ina field effect transistor can be applied to a trap-triggered fieldeffect transistor photodetector. In the example described below, anorganic field effect transistor is used, but other types of field effecttransistors can also be used.

Referring to FIG. 5A, a trap-triggered organic field-effect transistor(TTOFET) photodetector 300 uses the interactions of photon, carrier, andtraps in a light-trapping structure to produce a novel mechanism forhigh gain and low noise. The device uses the incident photon as aswitching valve to control the source-drain output current (I_(sd)). Thetrap-triggered organic field-effect transistor photodetector 300 allowsdramatic changing of the hole injection from the electrode into thesemiconductor channel by the trapped-electron-induced hole injection atthe organic semiconductor/electrode interface. Because the trappedelectrons are induced by the incident photons, each absorbed photon willcause a large output current change in the photodetector 300, resultingin a large apparent gain. The working principle of the photodetector 300is shown in FIG. 5A. In some examples, the photodetector 300 can have ahigh gain in excess of 10⁷ and a small noise current less than 1 nA atroom temperature.

FIG. 5B is a photo of a flexible, transparent organic field effecttransistor array.

The trap-triggered organic field-effect transistor photodetector 300 candetect weak light using the trapped-electron-induced charge injectionmechanism, which can be used to detect very weak light from ascintillator. This has applications in radiation detectors. For example,radiation detectors can be used to detect nuclear and radiologicalmaterials. A radiation detector may include a scintillator. In thescintillation detection process, high energy photons (such asgamma-rays) strike a scintillator material to emit ultraviolet(UV)-visible photons which are subsequently measured and amplified byphotodetectors. The trap-triggered organic field-effect transistorphotodetector 300 can be used to detect the photons emitted from thescintillator material. By using solid-state photodetectors that canoperate in room temperature, small, efficient, robust, and low costsingle radiation detectors and detector arrays can be fabricated. Thishas wide ranging applications in the field of homeland security. Thesolid state photodetectors can be powered by a low voltage, and canoperate unattended for long periods of time using battery or solarpower.

The trap-triggered organic field-effect transistor photodetector 300 canhave a performance comparable or superior to that of a photo multipliertube. The solid-state photodetector can be driven by a relatively lowbias voltage, and has excellent responsivity to the photon emission froma scintillator. The photodetector can be integrated with a low-costnanocomposite scintillator, enabling the detection and interdiction ofnuclear/radiological devices or component materials, allowing thedeployment of many compact, unobtrusive detectors in remote areas—suchas smuggling routes—where persistent manned surveillance may bedifficult.

In determining the performance of a photodetector, an important figureof merit is specific detectivity, which characterizes the weakest lightit can detect, or the sensitivity of the photodetector. The specificdetectivities (D*) of a photodetector are given by:

$\begin{matrix}{D^{*} = {\frac{({AB})^{1/2}}{NEP}\left( {{cm}\mspace{14mu} {Hz}^{\frac{1}{2}}W^{- 1}\mspace{14mu} {or}\mspace{14mu} {Jones}} \right)}} & \left( {{Equ}.\mspace{14mu} 8} \right) \\{{{NEP} = {\frac{{\overset{\_}{\iota_{n}^{2}}}^{1/2}}{R}(W)}},{R = {{EQE}/{hv}}},} & \left( {{Equ}.\mspace{14mu} 9} \right)\end{matrix}$

where A is the device area, B is the bandwidth, NEP is the noiseequivalent power, ι_(n) ² ^(1/2) is the measured noise current, R is theresponsivity, EQE is external quantum efficiency, and hv is the energyof the incident photon. A sensitive photodetector will have high EQE andlow noise for maximum signal-to-noise ratio.

The trap-triggered organic field effect transistor photodetector has ahigh sensitivity and can operate in room temperature due to the highhole mobility and low electron mobility of the organic semiconductor,the good insulating property of the polymer dielectric material, thelarge band gap of the semiconductor channel materialdioctylbenzothienobenzothiophene (C8-BTBT) (Eg=3.43 eV), and the highenergy barrier for the trapping of electrons in organic semiconductors.

Referring to FIG. 6A, in some implementations, a TTOFET 310 is based ona bottom-gate, top-contact organic field effect transistor (OFET). TheTTOFET 310 includes a source electrode 312, a drain electrode 314, agate electrode 316, a channel layer 318, and a dielectric layer 320. Awindow (not shown in the figure) in the photodetector allows light toshine on the channel layer 318. A controller (not shown in the figure)provides relevant bias voltages to the photodetector and reads outchanges in the channel current to determine the amount of lightdetected.

The device can be fabricated using an all-solution process. The gateelectrode 316 can be made of, e.g. ITO, and is covered by a lowtemperature cross-linked poly(4-vinylphenol) (PVP) dielectric layer 320.The organic semiconductor films are deposited in an nitrogen inertatmosphere on the PVP layer 320 from a C8-BTBT:polystyrene solution toform the C8-BTBT channel layer 318. Due to vertical separation of thesolution, a thin polystyrene layer 322 is formed below the C8-BTBTlayer. The C8-BTBT material is a p-type, air stable, small molecule,organic semiconductor having a mobility of about 5 cm²/Vs. Silver (Ag)source and drain electrodes 312 and 314 are thermally deposited with achannel length of 50 μm and a channel width of 1 mm, respectively.

FIGS. 6B to 6D show the photon detection process of the TTOFETphotodetector 310. The device operates in the depletion mode in which agate bias larger than the threshold voltage (V_(G)>V_(th)) is applied todeplete holes in the C8-BTBT channel layer. The injection of holes fromthe source electrode 312 is prohibited by the large energy barrier, asshown in FIG. 6B, which results in a low dark-current (noise). A featureof the TTOFET photodetector 310 is that the dark-current is reduced dueto the application of the gate bias voltage.

When the channel region is illuminated by UV light, excitons aregenerated in the C8-BTBT semiconductor layer. Some excitons willdissociate into free holes and electrons by the applied gate,source-drain electrical field or trap assisted exciton dissociation.Derived by the applied source-drain electrical field, the holes drifttoward the drain electrode immediately, while electrons are trapped inthe p-type C8-BTBT. The trapped electrons near the source electrode canincrease the channel current by inducing strong hole injection, asillustrated in FIG. 6D.

The trapped electrons on the source electrode side shift the highestoccupied molecular orbital (HOMO) of the C8-BTBT and align its HOMO withthe Fermi energy of the source electrode. The hole-injection barrier onthe source side then becomes so thin that the holes can easily tunnelthrough it. Thus, the electron-trapping layer acts as a photoelectronicvalve for hole injection. Incident photons can switch this valve “on”.If, on average, more than one hole is injected from Ag to the C8-BTBTlayer per absorbed photon, there is internal gain from the device.

Once the energy barrier for hole injection at the cathode becomesnegligible, the source contact changes from the Schottky contact intothe ohmic contact, which provides a large current injection. In additionto the trapped-electron-induced hole injection at the source electrode,the trapped electrons in the C8-BTBT near the dielectric interface alsoscreen the gate voltage and cause a shift of threshold voltage, whichfurther increases variation of source-drain output current (ΔI_(SD))upon light illumination.

Phototransistors can operate in photovoltaic mode or photoconductormode. For the photovoltaic mode, the gain is proportional to the photoinduced channel current with:

ΔI=g _(m) ΔV _(T)  (Equ. 10)

where ΔV_(T) is the photo-induced threshold voltage shift and g_(m) isthe transconductance. For the photoconductor mode, the gain is the ratioof the trapped electron lifetime (τ_(r)) and hole transit time (τ_(t)))through the channel layer:

G=τ _(r)/τ_(t), τ_(t) =d ²/μ_(h) V _(SD)  (Equ. 11)

where d is the channel length, and μ_(h) is the hole mobility inC8-BTBT. The TTOFET photodetector 310 combines both operation modes bythe dual functions of the trapped electrons. The trapped electrons inthe channel layer cause the photovoltaic effect, and the piling oftrapped electrons at the metal/organic semiconductor interface turn onthe photoconductor effect. The unique advantage of the TTOFETphotodetector 310 is that both photoconductive gain and photovoltaicgain are triggered on by the incident photons.

FIG. 7A shows the transfer curve of the TTOFET in dark and underillumination of 0.1 μW/cm². The device shows large variation in outputcurrent in the dark and under UV illumination. The off-current (darkcurrent) is less than 1 nA, which is limited by the measurementequipment. Under light, both threshold voltage shift and increase of thecurrent in the positive gated region (V_(G)>V_(th)) were observed,verifying the dual-mode operation. The gain of the dual modes can besummed, yielding a high gain of 10⁷ in these phototransistors, as shownin FIGS. 7B and 7C. The combination of the high gain and low noise canbe used to produce a very sensitive solid-state photodetector that canoperate at room temperature.

Referring to FIG. 8A, in some implementations, a TTOFET photodetector310 includes a p-n heterojunction formed by depositing a layer ofacceptor materials 332, such as C₆₀ or ZnO nanoparticles, on top of theC8-BTBT layer 318. Both C₆₀ and ZnO are good electron acceptors withdeeper (lowest unoccupied molecular orbital) LUMO than C8-BTBT, as shownin FIG. 8B. One additional advantage of adding the acceptor materials isthat both C₆₀ and ZnO are good UV light absorbers, and thephotogenerated excitons can dissociate at the heterojunction interface,contributing more free charges and increasing the absorption of light bythe photodetector. The thickness of the acceptor layers may be limitedto the exciton diffusion length in these materials.

Possible sources of the electron traps in the photodetector include thegrain boundary in C8-BTBT polycrystalline films, C8-BTBT damage causedby the thermal evaporation of Ag source/drain electrodes, and the —OHgroups on the surface of the dielectric material PVP.

Under weak light, the current flowing through the photodetector isdetermined by the electron injection from the Ag electrode to theC8-BTBT layer. Under the dark condition, the high energy barrier blocksany hole injection and is lowered by the trapped electrons upon lightabsorption. The trapped electrons lower the hole injection barrier bydoping the interface sheet, which has the same mechanism of the ohmiccontact by the high doping concentration in inorganic semiconductortechnology. The energy barrier change ΔΦ is a linear function of trappedelectrons (n_(t)), while the injection current follows an exponentialrelationship with the energy barrier change according to theRichardson-Dushman equation:

$\begin{matrix}{J \propto {\exp \left( {- \frac{\Delta \; \Phi}{kT}} \right)} \propto {\exp \left( {- \frac{n_{t}}{kT}} \right)}} & \left( {{Equ}.\mspace{14mu} 12} \right)\end{matrix}$

Therefore, there is gain due to the exponential dependence of injectedholes and incident photons. This model can be improved by consideringthe influence of trap distribution and lifetime, light intensity, andapplied bias on the current injection. The distribution of electrontraps along the out-of-plane direction may change the potential barrierprofile.

The grain size of C8-BTBT can be increased by optimizing thespin-coating parameters. For example, an off-center spin-coating (OCSC)method can be used to increase the mobility of C8-BTBT to a highmobility above 118 cm²/Vs. The highly crystalline C8-BTBT OFETs can beused for the TTOFET photodetector. In addition to the reduced electrontrapping lifetime, the high hole mobility may reduce the hole transittime, which may compensate partially for the lost gain due to the shorthole recombination lifetime (Equation 11). An alternative approach toincrease the response speed without comprising gain is to reduce thechannel length from the current 50 μm to about 1 to 10 μm, which canincrease the response speed by about 25 to 2,500 times (Equation 11).Thus, the TTOFET photodetector can have millisecond response time.

The mobility of 118 cm²/Vs is very high for organic semiconductors. Asan alternative to the organic semiconductors, graphene can be used asanother semiconductor channel material to form a hetero-planar structurewith C8-BTBT. Due to the deeper HOMO of C8-BTBT (5.7 eV) vs. graphene(4.5 eV), holes will transfer to graphene and transport through it. Thisenergy offset will also aid the dissociation of excitons in C8-BTBT.Compared to an organic semiconductor, graphene has several orders ofmagnitude higher carrier mobility (up to 200,000 cm²/Vs⁶⁷) and thus mayhave a faster response (>GHz). The graphene layer can be transferredonto the dielectric surface before the deposition of the organicsemiconductor. The injection of holes into graphene in dark conditionsis prohibited by the inserted C8-BTBT layer. Commercially availablegraphene layers on metal or dielectric substrates can be used. In someimplementations, to deposit the graphene layer on polystyrene, thegraphene layers can be transferred to PDMS stamps. After removing thetape chemicals, the graphene can be heat-transferred to the polystyrenesurface.

The lowest detectable light intensity for the photodetector isdetermined by the signal/noise ratio. The noise in this type of detectorcomes from the fluctuation of the channel current. To detect weak lightintensity, the induced I_(SD) by the absorbed photon must be larger thanthe noise so that a single photon is detectable. The noise in fieldeffect transistors is dependent on frequency and bias. There may bethree sources of electrical noise in a solid material: (1) thermal noise(or Johnson noise), (2) shot noise (or quantum noise), and (3)low-frequency noise (or flicker noise, 1/f noise, where f is thefrequency). Due to the high band gap of organic semiconductor materialsused (3.4 eV), the thermal noise is negligible. This makes the TTOFEToperable at room temperature. The quantum noise can be largelyrestrained by reducing the dark current. The dark current of the TTOFETdevice cannot be ignored even with a gate voltage of 0 V due to theunintended chemical doping or small charge injection, as shown in FIG.9. The dark current can be suppressed by depleting the channel regionwith a V_(G)>V_(T). The noise of an OFET photodetector, as shown in FIG.10, reveals that its low-frequency noise follows a power spectrum ofcharacteristics: 1(f)²∝f^(−α) with α=1.8. It is a typical characteristicof noise originating from the trapping/de-trapping of holes, most likelyat the grain boundary of the polycrystalline organic semiconductor inthe channel of the OFET.

Noise reduction can be achieved by using crystalline semiconductors andgraphene. By growing high-crystalline, large-grain-size organicsemiconductor single crystals and/or large area graphene flakes, thegrain trap density can be reduced. The thermal noise I_(SD) of an OFETdevice can be important if the device is operated at very highfrequency. Despite small thermal noise, the 1/f decays quickly with theincreased frequency, which limits the up-limit working frequency of thedetector by assuming a 1/f^(1.8) dependence. The thermal noise isdetermined by: I_(nd) ²=8kT g_(m)/3, where g_(m)=μ(V_(G)−V_(T))WC_(i)/Lis the transconductance of the transistor at a certain gate bias. If theTTOFET device works at the depletion region, the thermal noise can bereduced by carefully choosing V_(G)=V_(T), which can make the thermalnoise as low as 10⁻²² A/Hz. The reduced exponential factor with improvedcrystalline quality semiconductors may allow a higher speed operation ofthe TTOFET detectors.

In addition to the high sensitivity, a large active area is useful torecord the light emission from bulky scintillators. In someimplementations, a low-cost light waveguide concentrator—a simple coatedquartz glass—can be used to collect light from the scintillator. Inaddition to the low cost, such a light concentrator enables a compactintegration of bulky scintillators with the photodetector, and is morerobust compared to other mirror based concentrators. A slit can beetched on the center of the quartz glass by chemical etching (e.g. usinghydrofluoric acid), where the TTOFET photodetector is located. Since therefractive index of the organic materials (˜1.7) used in the TTOFET ishigher than quartz, the trapped light is efficiently coupled into theTTOFET photodetector. To prevent light loss, the edge of light trappingwaveguide glass and the scintillator crystal can be wrapped with ahighly reflective white film. One major merit of this light-trappingstructure is that it can be easily scaled up from a few cm² to 1,000 cm²with a very low cost.

For example, the waveguide concentrator can be a flat quartz coveredwith organic fluorescent dyes (e.g. polyfluorene derivatives) orinorganic phosphors such as LaMgAl₁₁O₁₉:Ce and BaSi₂O₅:Pb which have ahigh quantum yield (above 90%). These dyes or phosphors can convert theincident short wavelength UV (UVB) from scintillators, for example, 303nm from NaI:Ce, into relative longer wavelength UV-blue light UVA, 350nm for polyfluorenes. The re-emitted light goes all directions and thusredirects the incident light so that more light can be trapped by thewaveguide. Due to total reflection effect, most of the light will betrapped in the glass waveguide. The ratio of the trapped light (η_(tr))is η_(tr)=√{square root over (1−(n₂/n₁)²)}, where the n₁ is therefractive index of the DC UV glass (—1:55) and the n₂ is the refractiveindex of air (1.0). The trapping efficiency can be about 80%.

Other embodiments are within the scope of the following claims. Forexample, the p-type C8-BTBT material in the photodetector 310 can bereplaced by an n-type semiconductor material that is sensitive to lightand can trap charges.

What is claimed is:
 1. A method comprising: applying a voltagedifference between a source electrode and a drain electrode, andapplying a bias voltage to a gate electrode, to cause an electriccurrent to flow from the source electrode through a channel to the drainelectrode; generating one or more electric charges by using alight-absorbing material to absorb light and generate the one or moreelectric charges; trapping the one or more electric charges within thelight-absorbing material; and reducing the current flowing in thechannel by using the trapped one or more electric charges in thelight-absorbing material to influence charge carriers in the channel. 2.The method of claim 1 in which the channel comprises a P-typesemiconductor, and the bias voltage applied to the gate electrodecomprises a negative voltage.
 3. The method of claim 2, comprisingcounting a number of photons that have been detected based on a changein the current flowing through the channel.
 4. The method of claim 1 inwhich the channel comprises an N-type semiconductor, and the biasvoltage applied to the gate electrode comprises a positive voltage.
 5. Amethod comprising: providing a field effect transistor photodetectorthat comprises a source electrode, a drain electrode, a channelcomprising a light-absorbing material, a gate electrode, and adielectric layer disposed between the gate electrode and the channel;applying a voltage difference between the source and drain electrodes;applying a bias voltage to the gate electrode, in which initiallynegligible current flows in the channel when the bias voltage is appliedto the gate electrode; generating one or more electric charges in thechannel by using the light-absorbing material in the channel to absorblight and generate the one or more electric charges; confining the oneor more electric charges in the channel; and increasing a currentflowing in the channel upon confining the charges in the channel.
 6. Themethod of claim 5 in which providing a channel comprises providing achannel that comprises C8-BTBT.
 7. The method of claim 5, comprisingsensing a change in the current flowing in the channel when thephotodetector detects one or more photons.
 8. The method of claim 7,comprising determining an amount of photons detected based on the sensedchange in the channel current.